The Obama campaign has said if Republican presidential candidate Mitt Romney releases five years of tax returns, they will drop the issue.

Mr Romney, who has made public his 2010 taxes and plans to do the same with his 2011 returns, rejected the offer.

The former private equity chief said on Thursday he had never paid under 13% in taxes over the past 10 years, a much smaller rate than most US wage-earners.

Mr Romney will challenge President Barack Obama in November's election.

Obama campaign manager Jim Messina made the tax-returns offer to his counterpart, Matt Rhoades, in a letter on Friday.

'No more tax releases'

"Governor Romney apparently fears that the more he offers, the more our campaign will demand that he provide," Mr Messina wrote.

"So I am prepared to provide assurances on just that point. If the Governor will release five years of returns, I commit in turn that we will not criticize him for not releasing more - neither in ads nor in other public communications or commentary for the rest of the campaign."

Releasing several years of tax returns has become a standard move in recent presidential elections.

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President Obama has said he would raise taxes on the wealthiest Americans

And Mr Messina noted that the Republican candidate's father, former Michigan Governor George Romney, had released 12 years of his tax returns during his own unsuccessful run for the presidency in 1968.

Mr Rhoades rejected the offer in an email that began: "Hey Jim, thanks for the note.

"It is clear that President Obama wants nothing more than to talk about Governor Romney's tax returns instead of the issues that matter to voters, like putting Americans back to work, fixing the economy and reining in spending.

"If Governor Romney's tax returns are the core message of your campaign, there will be ample time for President Obama to discuss them over the next 81 days."